1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
In recent years, there is an increasing need for an image forming apparatus to have a higher processing speed and a better resolution and, accordingly, it is desired to improve a scanning speed of a scanning unit included therein. When a rotary polygon mirror is used in a high-speed optical deflection device, which is necessary to realize a high-speed scanning, for example, limitations exist for an increase in a rotational speed thereof.
By contrast, an image forming apparatus using a so-called “multi-beam scanning device” has been proposed to enable to scan a plurality of scanning lines simultaneously in a single scanning process by carrying out a scanning processing with a plurality of light beams that are output from a surface emitting laser having a plurality of light emitting points (a vertical cavity surface emitting laser, which is abbreviated as a VCSEL hereinafter). A method to realize a high-speed output of an image data is to make use of a plurality of beams for emission in such a way as the VCSEL does and, in particular, it has become general for a high-speed output apparatus to be equipped with a light source designed for multi-beam writing.
However, when the VCSEL is used as a light source unit in an optical scanning device included in an image forming apparatus, an intensity of a scanning beam may be altered by a temperature change or an aging variation to result in an uneven density in a final image (an output image). Thus, there is a widely known technique to suppress an appearance of an uneven density by performing Automatic Power Control (APC) that controls a drive signal of a light source based upon a result of a monitoring by detecting a part of the laser beams emitted by a light source of an optical scanning device with a monitoring element consisting of a detector such as a photodiode. (for example, see Japanese Patent Application Laid-open No. 2010-122473, and Japanese Patent Application Laid-open No. 2009-294327).
However, if the intensity of the monitoring beams received by the monitoring element is not strong enough, the monitoring element cannot detect the intensity accurately and, hence, the APC fails to function properly to make it difficult to maintain a light output to a predetermined value. As a result, the intensity of beam scanning becomes unstable to result in deterioration of the output image.
Furthermore, there is another problem in that the light output of the VCSEL is susceptible to reflected return light, and noise is easily generated by lights reflected by optical elements that are arranged in an optical path. Accordingly, the beam intensity fluctuates to cause an uneven density in the image. In other words, as illustrated in FIG. 12, in an optical system in which divergent light emitted by a light source unit 10 is converted into parallel rays by a coupling lens 24 and is collimated by an aperture 16, if a control treatment of a reflectance of the aperture 16 by, e.g., anti-reflection coating, is insufficient, weak light returns to the light source unit (VCSEL) to generate a noise.
As illustrated in FIG. 13, a method is known to solve the problem by arranging a quarter-wave plate 23 between the coupling lens 24 and the aperture 16.
Linearly polarized light that is oscillating by the linear polarization in a vertical direction on a surface of a drawing, as indicated by a symbol (a) in FIG. 13, is converted into circularly polarized light (b) after passing through a quarter-wave plate 23. Weak circularly polarized light (d) reflected on the aperture 16 passes through the quarter-wave plate 23 again, and is converted into linearly polarized light oscillating in a direction perpendicular to the direction of the oscillation thereof emitted by the light source (linearly polarized light (c) oscillating in a direction perpendicular to the surface of the drawing). Even if there is return light to the light source, light waves do not interfere with each other as long as the oscillation directions of light are perpendicular with each other and, hence, no noise is generated. Thus, by using the quarter-wave plate 23, the sensitivity of the VCSEL to the return light toward the light source can be reduced. Furthermore, as illustrated in FIG. 13, by rotating the quarter-wave plate 23 along an axis that is perpendicular to the surface of the drawing, it is avoidable for the light reflected by an incident face of the quarter-wave plate 23, i.e., a light source unit side, to return to the light source unit 10.
Typically, to prevent dust or the like from sticking to a light emitting element such as a semiconductor laser chip, the light emitting element is housed in a package member. The side thereof from which the laser beams are output is sealed with a piece of transparent glass or cover glass. Japanese Patent Application Laid-open No. 2009-294327 discloses the method to reduce the generation of aberration by rotating the quarter-wave plate along an axis that is perpendicular to the sub-scanning cross-section to achieve a small width for the aperture in the sub-scanning direction. It is assumed in the patent that, by setting (or adjusting) the relative positional relationship between the quarter-wave plate and the above-described transparent glass appropriately, the amount of the aberration can be further reduced associated with a simultaneous possible decrease in a reduction of the intensity. However, such a method is not disclosed in the patent.
Japanese Patent Application Laid-open No. 2010-122473 discloses a method with which it is possible, by using a light blocking member, to block a part of light beams that has passed through the coupling lens yet that is not used either in the beam scanning by reaching a target surface to be scanned or in the monitoring light beams to achieve downsizing in the aperture consisting of a pair of mirrors and an optical monitoring system without losing light use efficiency. With this method, however, there arises a problem in that the reduction in the intensity caused by transmission or reflection in each of the optical elements is unavoidable and, accordingly, the intensity of the laser beams incident on the target surface to be scanned or a monitoring sensor may be decreased.